---
title: "Pond Volume Calculator: Litres & Gallons | Pond Mosaic"
description: "How many litres or gallons is your pond? Rectangle, round, oval, sloping sides, shelves or irregular, with water weight and how long it takes to fill."
url: https://pondmosaic.me/pond-volume-calculator/
---

Litres, US gallons and UK gallons

# Pond volume calculator

How many litres is your pond? Multiply the surface area by the depth: a 3 × 2 m pond 0.6 m deep holds 3.6 m³, which is **3,600 litres, 951 US gallons or 792 UK gallons**. Pick your pond's shape for sloping sides, shelves or an irregular outline.

## How pond volume is worked out

- **Straight sides:** rectangle length × width × depth; circle π × radius² × depth; oval π × half-length × half-width × depth.
- **Sloping sides:** measure the top and the bottom. The prismoidal formula, depth ÷ 6 × (top area + 4 × middle area + bottom area), is exact for evenly sloping sides; the middle is the outline halfway down.
- **Shelf all round:** the full outline down to the shelf, plus the smaller inner part, shelf width in from each side, down to the bottom. For an oval this is a close approximation.
- **Irregular ponds:** average length × average width × the average of several depth readings. More readings give a better average.

One cubic metre is 1,000 litres. Gallons use the exact legal definitions: a US gallon is 3.785411784 litres and a UK (imperial) gallon 4.54609 litres, so 1,000 litres is 264.2 US or 220.0 UK gallons. Fresh water weighs very close to 1 kg a litre, so a 3,600-litre pond holds about 3.6 tonnes of water.

The fill time is the volume divided by the flow, from empty. Knowing the volume also helps size a pump and filter, dose treatments and plan how many fish a pond can hold.

## Sources

1. Wolfram MathWorld, [Prismatoid](https://mathworld.wolfram.com/Prismatoid.html): the prismatoid (prismoidal) formula, volume = h/6 × (A₁ + 4M + A₂).
2. NIST, [Handbook 44 (2026), Appendix C](https://www.nist.gov/document/2026-nist-handbook-44-appendix-c): 1 US gallon = 231 cubic inches = 3.785 411 784 litres; 1 foot = 0.3048 m exactly.
3. [Weights and Measures Act 1985, Schedule 1](https://www.legislation.gov.uk/ukpga/1985/72/schedule/1): 1 gallon = 4.54609 cubic decimetres (litres).
4. Blagdon, [Minipond filter FAQ](https://media.diy.com/is/content/KingfisherDigital/Marketplace/5015368010528_MNL_MP): pond volume = average length × average width × average depth × 1,000 litres; time how long a bucket takes to fill to measure a flow.
5. USGS Water Science School, [Water density](https://www.usgs.gov/special-topics/water-science-school/science/water-density): 0.99975 g/cm³ at 10 °C and 0.99802 g/cm³ at 21 °C, so “1 kg per litre” is within 0.2%.

## Pond volume questions

**How do I calculate how many litres my pond holds?**

Multiply length × width × average depth in metres, then multiply by 1,000. A 3 × 2 m pond with an average depth of 0.6 m holds 3.6 m³, or 3,600 litres.

**How do I measure an irregular pond?**

Measure the length and width in a few places and average them, then push a cane in at five or more points, shallow and deep, and average the depths. Multiply the three averages.

**How many gallons are in a cubic metre?**

About 264.2 US gallons or 220.0 UK gallons, because a US gallon is 3.785411784 litres and a UK gallon 4.54609 litres.

**How heavy is the water in a pond?**

About 1 kg per litre, or 8.35 lb per US gallon. A 1,000-litre pond holds roughly a tonne of water, which matters for raised ponds and decks.